Reactance tube circuit



March 25, 1952 C. W. CLAPP REACTANCE TUBE CIRCUIT Filed April 23, 1948INPUT OUTPUT FigS 35 /54 IN PUT ATTENUATOR CATHODE FOLLOWER REGULATEDB.+ POWER SUPPLY Inventor: Charles W.Clapp,

by M 23% His Attorney Patented Mar. 25, 1952 REACTAN CE TUBE CIRCUITCharles W. Claim, Schenectady, N. Y., assignor to General ElectricCompany, a corporation of New York Application April 23, 1948, SerialNo. 22,818

6 Claims. (01. 178-44) My invention relates to tube circuits wherein atube is arranged to supply an alternating current to an external circuitsuch that the current thus supplied is derived from and bears apredetermined relationship to a signal voltage appearing in suchexternal circuit. One type of such tube circuits is generally referredto as a reactance-tube circuit, and my invention is particularlyapplicable to this type of circuit wherein an electron tube simulates areactance element.

My invention also concerns filter networks comprising a simulatedreactance element in the form of an electron discharge device.

An object of the invention is to provide, for use in electric circuits,an improved electronic reactance element, or an element simulating acombination of impedance elements.

A more specific object is to provide a reactance element, adapted to usein filter networks, oscillator circuits, and the like, which acts as asubstantially pure reactance over a wide frequency range.

A further object of the invention is to provide an improvedreactance-tube circuit which will not load the circuit associatedtherewith, and which will have a high effective Q.

Reactance tubes have provided convenient means in filter and oscillatorcircuits, and the like, for varying the filter or oscillator circuitcharacteristics and are often used to replace relatively bulky andexpensive imnedances. A particular difficulty common to such prior artdevices has been associated with the excitation of the reactance tubewherein the direct current circuits necessary to supply the anodeoperating potential to the tube have caused a load to be placed on theanode which supplies the reactive current and have thus loaded thecircuit to which the anode is connected.

It is a general object of the invention to remove or greatly reduce thisloading effect of reactance tubes.

Further objects and advantages of my invencircuits according to myinvention illustrating certain possible variations of the circuits ofFigs. 1-3.

The diagram of Fig. 1 is illustrative of a bridged-T networkincorporating as an operative portion a reactance-tube circuit accordingto the invention.

In Fig. 1 the bridge with designated input and output terminals iscomposed of two series capacitors l and 2 bridged by a variable resistor3 and, connected between a point 4 at the juncture between thecapacitors and the grounded lower conductor, an inductance element. theinductance element being a reactance-tube circuit in accord with theinvention.

The reactance-tube circuit comprises areactance tube 5 of which theanode 6 draws a current substantially in phase quadrature with thesignal voltage at point 4. The signal on anode 6 is produced as a resultof the application on control electrode 1 of the signal voltage at thejuncture 4. The signal voltage is applied through capacitor 8 to thecontrol electrode 9 of triode tube In, whereby the signal, shifted inphase by substantially degrees, appears across capacitor II in thecathode circuit of tube Ill. The phase-shifted signal on capacitor I lis furnished to control electrode 1 by coupling capacitor I2. Variableresistor l3 in the cathode circuit of tube l0 cooperates with capacitorH to form the phase-shift circuit, and by varying either the resistancevalue or the capacitance value in this circuit, or both of them, thefrequency response of the circuit may be changed. Resistor l3 andcapacitor II are each shown as variable for this purpose.

Anode operating potential for tube 5 is obtained through theanode-cathode path of tube I ll. Thus a suitable source of 3+ potential,battery I4, is connected directly to anode l5 of tube Ill, and cathodeI6 is connected through a biasing resistor I! for tube Ill and, inseries therewith, a relatively high value resistor ill for supplying thedirect current to anode 6 of tube 5. The cathode IQ of tube 5 isreturned to the negative terminal of the source [4 through a suitabledegenerative resistor 20 to improve the linearity between anode currentand the voltage applied to the control electrode. This resistor may bevariable if desired, and for increased value will increase the value ofthe simulated inductance. The return for control electrode 9 of tube I0is through a leak resistor 2| to the juncture of resistors l1 and I8,and the return for control electrode 1 of tube 5 is "anode ,6 of tube 5.

through a leak resistor 22 to the cathode l9 or to a' tap on resistor20.

In operation it will be found that the alternating current voltageappearing on the cathode Hiof tube ID, as well as the voltage at theadjacent end of resistor I3, will be substantially equal to the voltageat the juncture 4 of capacitors 1,2. The resistor l8, therefore, offersminimum loading of the filter network. Resistor [8 may have a resistanceof the order of 100,000 ohms and as mentioned, carries very littlealternating current. The alternating-current voltage across resistor l8may be, for instance, about 2 percent of the voltage between terminal 4and ground, depending on the closeness with which cathode It follows thecontrol electrode 9 and the small drop in resistor H. The alternatingcurrent drawn by the anode of tube 5 is the desired quadrature current,and the current thus drawncauses the reactance-tube circuit comprisingtubes 10 and 5, of which tube 5 is, of course, the reactance tube, tofunction, with respect to the condenser junction, as a high Qinductance. It will be understood that the input. impedance to thecontrol electrode of tube I is very high since this tube operates as acathode follower.

.The reactance-tube circuit of Fig. 2 is similar to the circuitconnected to juncture 4 of Fig. 1, but in addition to its being modifiedto use pentode tubes (a modification that may be readilyaccomplished, ofcourse, in the circuit of Fig. l) by the addition of screen electrodevoltage dropping resistors 23, 24 and bypass capacitors 25, 26, asshown, and by adding suppressor elec trode connections to the respectivecathodes, therehas been provided a cathode load resistor '21 for tube l0and a-coupling capacitor 28 for providing :the signal generated acrossresistor 2'! by thecathode follower operation of tube It to the end ofresistor 18 at a point remote from Resistor 29 provides direct currentfrom the source M to operate tube 5. Thus 'B+ operating potential fortube 5 is not drawn through the-anode-cathode path of tube l0. ,Also inFig. 2, cathode resistor 28 of tube 5 is'shown as comprising twoportions with the control electrode return through resistor 22 madepoint 4 is fed through coupling capacitor 8 to the control electrode 9of tube [0 producing a signal similar to that at point 4 on cathode H3,across resistors I1 and 21 in series. This signal is subjected to aphase shift in the capacitor-resistance network l l, I3'for applicationto control electrode I of tube5 through coupling capacitor l2. The

signalfrom cathode I6 is applied without phase shift to a point on theresistor 18 such that the signal will not be grounded through thepotential supply [4, being isolated therefrom by resistor 2s.

'.The application of this signal as described prevents ithe flow ofsubstantial alternating current through resistor l8 since the oppositeends thereof are at substantially the same alternating cur rent voltage.

There is, accordingly, very little loading to lower theQ ofthe-simulated reactance. The circuit of Fig. 2 though using a few more'componentparts than the circuit of Fig. 1, may be better adapted tocertain applications where, for instance, tubes 5 and ID are designedfor anode currents of different values. The Q of the 4 circuit of Fig. 2may be slightly less than that of Fig. 1 modified to include pentodetubes.

Fig. 3 is a schematic diagram of a distortion and noise analyzer adaptedfor use in testing amplifiers, as for instance audio amplifiers, anddemonstrates a particular application of my invention. The instrumentcomprises, preferably, an input attenuator with suitable input terminalsfor providing an input signal of adjustable volume or strength to asuitable wide band cathode follower or amplifier stage 3!. The output ofcathode follower stage 3| is fed through a bridged-T network, indicatedgenerally at 32, of the type incorporating my invention and whereinsignals of a desired frequency are greatly attenuated while signals ofall other frequencies, including all harmonics and sub-harmonies of theblocked frequency, are passed with very little attenuation. The signalspassed by the filter are amplified in a wide band amplifier 33 and themagnitude thereof is measured or indicated by an alternating currentmeter 36, which maybe calibrated in decibels if desired.

In utilizing the analyzer of Fig. 3, the output terminals of anamplifier to be tested are connected to the input terminals ofattenuator 3t, and a signal of a desired single frequency is supplied tothe input of the amplifier under test by a high-quality audio signalgenerator with low noise voltage. The amount of noise and distortiongenerated by the amplifier under test is indicated on meter 34 when thefilter network 32 is adjusted to block the single frequency of the audiosignal generator.

A switch is arranged intermediate to amplifiers SI, 33 to disconnect andto bypass filter network 32 for the purpose of calibrating meter 35,whereby the full output of the audio signal generator may be measured,as well as the output of the amplifier under test, without attenuationof any frequency. The resultant readings provide an indication ofamplifier gain. The instrument may also be utilized with the switch 35in the position shown in the figure with the filte'r network in thecircuit to test the audio signal generator output directly fordistortion or noise components. If such components are present in thesignal generated, allowance for their presence should be made indetermining amplifier characteristics.

The-bridged-T network 32 as used in the practical instrument of Fig. 3comprises a triode tube ID as a cathode follower and a pentode 5 as thereactance tube. A voltage regulated rectifier power supply 35 is usedfor supplying 13+ operating potential, and ganged switches are arrangedto select suitable frequency ranges by switching desired capacitors land 2 and corresponding resistors IS.

The signal from amplifier 3| is applied through a selected capacitor lto switch armature 37, through conductor 38', representing the juncture4 between the capacitors of the filter network of Fig. 1, through aselected capacitor 2 and switch armature 39' to amplifier 33. Seriescapacitors l, 2 are bridged by a resistor comprising a fixed portion :20and variable portion 3. It will be understood that this resistorcompensates for negative resistance characteristics of a T filterconsisting of series capacitors and a parallel inductance.

The signal from conductor 38 is applied through coupling capacitor 8 tocontrol electrode 9 of triode tube ID. The signal which appears oncathode i6 is shifted in phase by the network including capacitors l land a selected one of re- I 6 to the end of resistor I8 remote fromanode 6 whereby iii-phase current loading of conductor 38 is prevented,and only the desired quadrature current is supplied to conductor 38 byanodefi of tube 5.

The cathode circuit of tube 5 includes an upperv fixed portion ofresistor 20 connected to the cathode and bypassed by a large capacitor4!.

The control electrode is returned through resister 22 to the lower endof this bypassed selfbiasing portion. The lower portion of resistor 20comprises an additional fixed portion, and a variable portion forpreliminary frequency adjustment. The variable portion of resistor 20may be preset, together with one of the variable capacitor elementscomprising capacitor II, to properly calibrate the frequency controls ofthe instrument. As heretofore explained, increas ing the value of thisresistor increased the value of the simulated inductance. The otherelement of capacitor II is one of the main tuning controls and operatesto vary the frequency characteristics of the filter within the bandsselected by the. position of ganged switch armatures 31,

- 39 and 42, the armature 42 being used to select the desired resistor I3, and armatures 3'! and 39, to select the desired capacitors l and 2,respectively.

In practice. tube I may be one-half of a type 68L? dual-triode tube. theother half of which may be conveniently used in cathode-followercoupling device 3!, and tube may be a type BSJ'I; To tune the filternetwork from 50 to 150 cycles, capacitors I and 2 of .01 microfarad maybe selected with a resistor I 3 of megohms.

. Assuming a main tuning capacitor II variable from 200 to 4200micromicrofarads. bypassed by a preset trimming capacitor of about 25micromicrofarads, tuning of the main tuning capacitor, the range of 50to 150 cycles will be completely covered by varying the main tuningcapacitor. Resistor 3 may be of 100,000 .ohms.

- maximum, and resistor 40 may be a fixed resistor of 56,000 ohms. Thenegative resistance of the upon the Q of the reactance andcharacteristics comparable to those obtainedby the circuit of Fig. 3 arealmost unobtainable with practicable would be of the order of 500henries. In addition; a large number of. coils would haveto be providedto cover a range of fill-15,000 cycles,

' since the inductance 05,000 cycles is about 0.6 'henry. In a reactancetube according to the prior art, wherein the. B+ operating potentialnetwork throughout this frequency range may be properly compensated bysuitable adjustments of resistor 3 to obtain optimum blocking of thfrequency to be blocked. I

A frequency range of 150 to 500 cycles may be obtained by movingarmatures 31, 39 and 42 to select capacitors I and 2 of .003 microfaradcapacitance each and a resistor I3 of 5 megohms/ A range of 5000 to15000 cycles is obtained by selecting capacitors I and 2 of .0001microfarad each and a resistor 13 of 150,000 ohms. Intermediate rangesbetween 500 and 5000 cycles are obtained by selecting suitableintermediate values of these elements. By way of further example, thefilter will reject signals of 1000 cycles with capacitors I and 2 of.001 microfarad each. a resistor E3 of 1.5 megohms, with resistors 3and. 40 providing a total series resistance of approximately 125,000ohms and with the main-tuning capacitor II adjusted to approximately 900micromicrofarads.

The Q of the reactance simulated by tube 5 at any frequency between and15,000 cycles is approximately 6, though a Q of 15 is obtainable for asingle fixed frequency by a careful selection of circuit constants. Asharp rejection characteristic for the filter is dependent is supplied.to a load resistor connected to the anode of the reactance tube. andthus to the circuit using the reactance effect, the Q is of the order ofhalf of that obtained by a circuit in accord with Fig. 3, sincejltheload resistor is a relatively low impedance; for the alternating currentvoltage, and the resultant rejection characteristic of the filter is}substantially broader due to the lower Q.

In effect, tube II! can be consideredas an infinite impedance forsignals at juncture 4. This infinite impedance prevents any loading ofthe juncture 4 by the low impedance power supply 35.

Figs. 4 and 5 disclose portions of circuits according to the inventionin which tube 5 simulates, at a point 4. a capacitive reactance (Fig. 4)and'a series resonantcircuit (Fig. 5). It will be understood that thecircuits of Figs. 4 and 5 are not complete but are intended to indicatepossible variations of thejgcircuits of Figs. 1-3. In Fig. 4., thesignal supplied through capacitor! and appearing on cathode I6 of tube IDis applied to the end of resistor I8 remote from anode 5. of tube5whereby, as inthe circuits of previously discussed figures,substantially no alternating current flow in resistor I8. The phaseshift circuitcomprises a resistor I3 and an inductance 43. rather than acapacitoij I l as heretofore. and provides a phase-shiftedsignal tocontrol electrode 1 of tube 5, through'icapacitor I2, such that tube 5supplies alternatingcurrent to point 4 leading the voltage of point 4by'substantially degrees for a wide band of frequencies. Inductance 43may be made fixed oryariable as shown. as may be resistance l3,asjdefsired, to alter the value of the capacitance simulated by tube 5,and the value may be changed or preset by varying the cathode resistorof tube 5 in Fig. 3. I

In Fig. 5, the capacitor I I of Figs. 1-3 is replaced by a paralleltuned circuit 44, comprising a parallel connectedpapacitor andinductance. The capacitor is shown as variable. though, of course,either, neither or both the capacitor and inductance may be avariable asdesired. As is the case in Fig. 4, elements or portions ofthe cirsuit ofFig. 5 functioning in the same or similar manner to correspondingelements orportions of the circuit of Fig. 1 bear the same referencenumerals. 1

While Figs. 1 and 3 disclose an inductive reactance simulating circuitaccording to the invention used in a bridged-T network, it will beunderstood that the bridged-T network itself iorms no part of thepresent inventionzexcept insofar as the novel reactancetube circuitpossesses particular advantages when used in a network of'the typeshown. On theother hand, the reactancetube circuits of Figs. 1, 2, 3 and4 are of general applicability as, for instance, in other types ofnetworks or in oscillator circuits and the like,- or in otherapplications where it may be desired to simulate a frequency responsiveelement or elements. Ii-fit be ,desiredto simulate a parallel- "tunedcircuit, a-series-tuned circuit may be substituted for circuit 44 ofFig. 5, and other modifi- :cations will be apparent'to those skilled inthe art. In general, itmay be said that the imped- 'c.nce simulated bytube will be, the reciprocal 'oi' the impedance of theelement, orcombination oi-elements; in series with resistor l3, times aic'onstantdeterminedby the tube and circuit constants. Resistor ['5should be of high value in comparison-with the impedance in seriestherewith il-this relationship is to be maintained. It "is practicableto substitute other types of phase shift circuits in the cathode circuitof tube ID. in Fig. 4; for-example, resistor 13 may be replaced any ahigh impedance capacitor and inductance :43' replaced by a'lowresistance, whereby tube 5 stiil-simulatesacapacitor.

"z Accordingly,- while a specific embodiment has been shown-anddescribed, it will, of course, be u'nderStoodthatvarious modificationsmaybe made without departing from the-invention. The appended claimsaretherefore intended. to cover -g'iiy suchi'm'odifications-within the truespirit and scope orthe invention.

' What I claim 'as new and desire to secure by Letters'Patent of theUnited States is:

LA reactance-tube circuit for simulating a reactance'between a referencepoint and a second point having an alternating voltage thereon, said-circuit comprising a cathode follower discharge "device with a controlelectrode, means for sup- "plying the alternating voltage-at said second'point to said control electrode whereby substantially'thesame voltageappears on the cathode of i "saiddischarge" device, phase shifting meansconi nected to said.v cathode for providing a voltage ifted iiiphase'from said cathode voltage; means applvsaid phase shittedvoltagentothe, control "electrode of .a' second discharge device, an anode in saidsecond device, an'impedance in the anode 'circuit of said second device,means for furnish- 'in'g said cathode voltage to said *impedance re"meterm said anode, 'and alternating current 'co'nductive meansconnecting said anode to said- '2?- A i-e'a'ctance tuhe'circuitcomprising a cathode 'fo'llow'cr electi'ondischarge devicewith acont-r01" electrode, means for impressing on said conmoi-electrodeafirst signal for which the reactance is to be efiective whereby saidsignal is reproduced on the cathode of said device, a phase shiftcircuit connected to receive-the reproduced from the cathode of saiddevice and pro- "ducing a phase shifted signalpa second electrondischarge device with an anode, a cathode, and a control *el'ectrode'connected to receive said -phase shifted'signahan impedance connected tosaid anode, and electric. conducting means con- 'nected between thecathode of said first discharge "device'and' a point on said impedanceremote from said anode for applying said reproduced signal from saidcathode of said first discharge dcvice tosaid impedance whereby ananodecathode circuit-of said second discharge device .lraws:. cu rre11ti i-quadrature to said first signal a but imposes substantially noin-phase-load for a reactance efiectiveconnection withsaid first sig--nal.

'- -3. In combination, an alternating current cir- 'witlrananode, acathode and a controlelectrode. ananode-cathode circuit for said devicecomprising an impedance element; a phase shifting circuit responsive tosaid second voltage for providing a third voltage differing in phasefrom said second voltage and connected to impress said third voltagebetween said control electrode and said cathode, means coupling saidanode to said point, whereby a current corresponding to said thirdvoltage is'supplied to said point, and electric conducting meansconnected to receive said second voltage and to impress said secondvoltage on said impedance element to'alter the reactive effect of saidanode-cathode circuit current at said point.

4. A simulated inductance comprising an electron-discharge device withan anode, a cathode and a control electrode, and having an anodecathodecircuit comprising in series an impedance and the anode-cathode path ofa second electron discharge device and a low impedance source of directcurrent operating potential, a control electrode in the anode-cathodepath of said second discharge device, one end of said impedancecomprising one terminal of said simulated inductance, means for applyingthe signal appearing at said one end to the control electrode of saidsecond device, phase shift means connected between the other end of saidimpedance and the control electrode of said first mentioned dischargedevice for applying a signal in phase quadrature to. the signal at saidone end to the control electrode of said first mentioned dischargedevice whereby the alternating current flowing through said firstdischarge device is in phase quadrature to the voltage appearing at saidone end, the other terminal of said simulated in- H ductance beingconnected for alternating currentto a point of said anode-cathodecircuit in low alternating current impedance relationship to a terminalof said source of operating potential.

shift circuit connected to receive the reproduced signal from thecathode of said device and :to produce a phase shifted signal, a secondelectron discharge device with an anode, a cathode, and a controlelectrode connected to receive said phase shifted signal, an impedanceelement connected to said anode, a degenerative impedance connected tothe cathode of said second device, and means applying said reproducedsignal from said cathode of said first device to a point on saidimpedance element remote from said anode, whereby the anode-cathodecircuit of said second discharge device imposes substantially no inphaseload for a reactance effective anode connection to said first signal andwhereby said second device simulates a 'reactance for said first signalof value determined in part by the value of said degenerative impedance.

6'. A reactance-tube circuit comprising a cathode follower dischargedevice with an anode, a

cathode and a control electrode, a phase shift circuit connected to saidcathode, a second discharge device with an anode, a cathode and acontrol electrode, said last control electrodebeing coupled to saidphase shift circuit, an impedance connected from said anode of saidsecond device to the cathode of said first device,

an alternating voltage conductive connection 9 from said anode of saidsecond device to the control electrode of said first device, and apotential source with a positive terminal connected to the anode of saidfirst device and a negative terminal connected to the cathode of saidsecond device, 5

whereby said second device simulates a reactance for voltages appliedbetween its anode and a terminal of said source.

CHARLES W. CLAPP.

10 REFERENCES CITED The following references are of record in the fileof this patent:

Article: "A New Type of Practical Distortion Meter, Proceedings of theI. R. E., vol. 31, No. 3, March 1943, pages 112-117. (Copy in17844-1911)

